PRITCHARDS WEDGE PHOTOMETER. 



317 



Extract from Original Record. 



Conditions the same as on the previous day, August 9, except the following : — 

 Temperature of Apparatus = 30 C. at 2 p.m. 

 State of Sly, milky blue with cumuli. Good sky between clouds. 

 Setting on D„ = 315° 47' 15". (Slit = 0° 0' 0".) 

 JBattery Current = 0.038 Ampere. 



Object = repetition of measurements of wedge transmission at different points in the spectrnm. 

 Time of first five series 10.15 a.m. to 12.30 p.m. 

 " last " " 12.30 P.M. to 1.45 p.m. 



In our measures in the direct beam each series was complete in itself, and hence 

 we were independent of any changes of the weather from day to day; but here, 

 though the series as between different points of the wedge may be also complete 

 in itself, it is not necessarily true that the comparison between different wave- 

 lengths made on different days, though at the same point of the wedge, is to be made 

 with equal immunity. 



Owing to variations in the initial solar radiation, due to atmospheric changes 

 and the alteration of atmospheric absorption, with the changing altitude of the sun, 

 the foregoing series as represented by the vertical columns are not then so strictly 

 comparable with each other ; nor, rigorously speaking, can one day be exactly com- 

 parable with another when the progressive absorption in the spectrum is in question, 

 rather than the rate of absorption for different parts of the wedge. The following 

 continuous series, made when the sun's altitude above the horizon was 54°, with the 

 wedge set at 0.3 in., will however serve to indicate the relative intensities for one 

 position. 



TABLE VI. 

 Wedge at 0.3 in. 



There is of course no reason why similar series should not be taken for every 

 part of the wedge, except the inordinate time demanded in waiting for days of 

 unexceptionable clearness ; for experience seems to show that with all its drawbacks 

 sunlight is better for this purpose than artificial light. 



